1. Field of the Invention
The present invention relates to a hard disk drive, and more particularly, to a method of testing quality of servo burst signal recorded on the hard disk drive and a recording medium and apparatus adapted therefor.
2. Description of the Related Art
A hard disk drive (HDD) is a magnetic recording device storing information. Information is recorded on concentric tracks, which are formed on a surface of the disk. A spindle motor rotates the disk and a read/write head accesses information on the disk. The read/write heads are mounted on an actuator arm, which is rotated by a voice coil motor (VCM). The voice coil motor is activated by current and rotates the actuator, resulting in movement of the heads. When the hard disk drive operates, the read/write heads must be arranged precisely on the tracks of the disk in order to assure information read/write operations.
A servo control circuit generally controls a position of the head. Such a servo control circuit detects and controls the position of the head using burst signals that are recorded on the disk.
In order for the head to follow the tracks accurately, servo data containing burst signals must be recorded on the tracks. Servo track writing (STW) is a process of magnetically recording such servo data on the disks.
In order to accurately control the position of the head, the quality of the burst signals recorded through the STW process, that is, intensity and phase of the recorded burst signals, is important.
In one method of testing write quality of servo burst signal, positive track width and negative track width are measured and, whether the measured track widths exceed predetermined values is checked. Here, the amplitudes of the burst signals indicate intensity of electric signals generated from the head, corresponding to magnetic field strengths of the recorded burst signals.
The track width indicates a burst signal value at the position where two burst signals intersect, the positive track width indicates the track width in a positive direction (one side around a center of a track), and the negative track width indicates the track width in a negative direction (the other side around a center of a track). If burst signals are correctly recorded, the burst signal value will be ½ of its peak value at the position where two burst signals intersect. Accordingly, by measuring the burst signal value at the position where two burst signals intersect, quality of recorded burst signal can be verified.
Specifically, in a conventional method for testing a servo burst signal, when a number of tracks, whose positive track width or negative track width is larger than a predetermined threshold value, exceeds a predetermined value, write quality of the servo burst signal is determined to be low or weak, and therefore, STW is performed again. In other words, according to a conventional method of testing the servo burst signal, quality of the servo burst signal is determined by comparing only intensity of the servo burst signal.
Meanwhile, with the necessity for large-capacity hard disk drives, new STW technologies have been developed for overcoming problems associated with the performance of the hard disk drive, that is, technologies have been developed for reducing process time.
One STW technology is to record servo data in two stages. According to this technology, a reference servo signal is recorded on the disk, and then, a final servo signal is recorded on the disk based on the reference servo signal. Here, the recorded reference servo signal having relatively wide width (for example, several tracks) is used as a basic unit. Servo control is performed by the reference servo signal and the final servo signal is recorded on the respective tracks.
Generally, the reference servo signal is recorded in a clean room and the final servo signal is recorded outside of the clean room.
However, new types of problems occur in such new STW technologies. Two typical problems will be described below.
Problem 1. Although the hard disk drive has no errors in a conventional servo burst signal testing method, data recorded in offtrack over wide areas poses a problem.
FIG. 1 illustrates measured track widths. In FIG. 1, there are shown track widths that are measured with respect to test tracks. Each illustrated box illustrates zones on the disk, with a numeral under each box indicating the corresponding track number of a central track of the zone. The lower waveforms show amplitudes of positive track widths and the upper waveforms show negative track widths. Referring FIG. 1, “P Over Cut” indicates the number of cases when positive track width is larger than a predetermined threshold value and “N Over Cut” indicates the number of cases when negative track width is larger than a predetermined threshold value. Both “P Over Cut” and “N Over Cut” are zero, which means that the burst signals are recorded on the respective sectors of the tracks with normal intensity.
However, when data recorded on the hard disk drive having the measured result of FIG. 1 is recorded in the offtrack, it is observed that the data is recorded over wide areas, as shown in FIG. 2.
FIG. 2 illustrates an example of data recorded on the hard disk drive.
In FIG. 2, three tracks are shown and white-colored portions indicate recorded status of data. Referring to FIG. 2, data is recorded not around a center of the track but in −30% offtrack. Here, the percentage is represented as a ratio for track pitch.
It is known that this phenomenon is caused due to different amplitudes of burst signals among the tracks. Accordingly, it is understood that error in recorded burst signals may not be correctly detected by a conventional method for testing a servo burst signal.
FIG. 3 illustrates an example of burst signal profiles on one track.
In FIG. 3, the abscissa axis indicates tracks and the ordinates axis indicates amplitude of burst signals recorded on the disk. Referring to FIG. 3, it can be seen that amplitudes of the respective burst signals are repeatedly increasing and decreasing periodically over the tracks. This means that the track width is periodically increasing and decreasing. This effect causes data to be recorded offtrack.
Problem 2. Although write operation is performed by normal write parameters, adjacent tracks are erased.
FIG. 4 illustrates another example of data recorded on the hard disk drive.
In FIG. 4, there are shown three tracks and the white-colored portions indicate recorded status of data. Referring to FIG. 4, it can be seen that the lower portions of an upper track are regularly cut away, and upper portions of a lower track are regularly cut away. This means that some of the data recorded on the upper and lower tracks is erased due to adjacent track interference, when data is recorded on a central track.
This phenomenon is caused because specific burst signals C and D are shifted in batches.
FIG. 5 illustrates another example of burst signal profiles on one track. Referring to FIG. 5, while burst signals A and B are recorded normally, burst signals C and D are recorded while being shifted from normal positions.
In other words, although the measured results of track widths are normal in the respective track, the burst signals are recorded shifted from normal positions. For this reason, one track becomes narrower than a normal track and the adjacent tracks become wider than normal, thus resulting in adjacent track interference.